Earth engaging drill bits are used extensively by industries including the mining, oil and gas industries for exploration and retrieval of minerals and hydrocarbon resources. Examples of earth-engaging drill bits include fixed cutter drill bits (“drag bits”).
A drill bit wears when it rubs against either of an earth formation or a metal casing tube. Drill bits fail. A cooling and lubricating drilling fluid is generally circulated through the drill bit using high hydraulic energies. The drilling fluid may contain abrasive particles, for example sand, which when impelled by the high hydraulic energies exacerbate wear at the face of the drill bit and elsewhere.
Drill bits may have a body comprising at least one of hardened and tempered steel, and a metal matrix composite (MMC). A steel drill bit body may have increased ductility and may be favorable for manufacture. A steel drill bit body may be manufactured from a casting and wrought manufacturing techniques, examples of which include but not limited to forging or rolled bar techniques. The steel properties after heat treatment are consistent and repeatable. Fracture of steel-bodied drill bits are infrequent; however, a worn steel drill bit body may be difficult for an operator to repair.
A MMC generally but not necessarily comprises a high-melting temperature ceramic, for example tungsten carbide powder, infiltrated with a single metal or more commonly an alloy, for example copper or a copper-based alloy, having a lower melting temperature than the ceramic powder. MMC's may be made using a premixed powder comprising a metallic powder and a ceramic powder. The premixed powder may be a cermet powder. FIG. 1 shows a light microscopic micrograph of a prior art MMC 1 prepared using metallographic techniques.
The MMC 1 consists of two principle phases. The soft phase 2 is formed through liquid metal infiltration of hard particles 3. The soft phase 2 is in the as-cast condition. Soft phases 2 may be considered as those that are significantly softer than the hard particles 3 and may be classified as having resistance to localized indention less than 1,000 HV, and even less than 250 HV. The elastic modulus of the soft phase 2 is also much lower than that of the hard particles 3.
The hard particles 3 are generally metal carbides, borides or oxides, for example tungsten carbide, tungsten semi-carbide or cemented carbide. The hard particles 3 typically have a resistance to localized indentation greater than 1,000 HV. The hardness of WC (tungsten mono carbide) is 2,200-2,500 HV. Between the soft phase 2 and hard particles 3 there is an interface 4 at which is a bond between the hard particles 3 and the soft phase 2. The bond is in the form of an inter-atomic diffusion of species between the hard particles 3 and soft phase 2. Interfacial strengths may be high due to chemical compatibility. The hard particles 3 act to stiffen, and strengthen the resulting MMC 1 relative to the soft phase 2 alone.
A MMC drill bit body may wear more slowly than a steel drill bit body. MMC drill bit bodies, however, more frequently fracture during casting and/or processing and/or use from thermal and mechanical shock. Fracturing may cause an early removal of a drill bit from service because it may be structurally unsound or have cosmetic defects. Alternatively, the MMC drill bit body may fail catastrophically with the loss of part of the cutting structure, which may result in sub-optimal drilling performance and early retrieval of the drill bit.
In many cases, it is a wing or blade of a drill bit that fractures. Wing or blade failures are economically damaging for drill bit manufacturers. Occurrences on a weekly or monthly basis may impact profitability and reputation. Were a drill bit manufacturer making 300 bits per month, with 1 in every 1,000 bits failing, a fracture event would occur on average approximately once every three months—this may be considered too frequent. One fracture for every 10,000 bits, while still not ideal, may improve the drill bit manufacturer's profit and reputation.
MMCs are generally considered to be a brittle material. Samples from a population of a brittle material objects exhibit strength variations because of unique flaws and defects. The strength of a sample of a MMC may be determined using a Transverse Rupture Strength (TRS) Test, where a load is centrally applied to a cubic or cylindrically shaped MMC sample that is supported between two points. A plurality of samples may be tested to derive a mean strength and a standard deviation of applied stress at the moment of rupture, which are then taken as being representative.
The retrieval of a worn or failed drill bit from a drilled hole, for example a well or borehole, is undesirable. The non-productive time required to retrieve and introduce into the drilled hole a replacement drill bit may cost millions of dollars. Drill bits and other earth-engaging tools with increased wear resistance and lower rates of failure may save considerable time and money.